Method of chromium plating



Sept. 12, 1939.

DEGREES CENTIGRADE M. BROWN ET!" AL METHOD or c'u'izouwu PLATING FiledSept. 11, 1936 ATM/MEX Patented Sept. 12, 1939 UNITED STATES PATENTOFFICE METHOD OF CHROMIUM PLATING New York Application September 11,1936, Serial No. 100,284

3 Claims. (Cl. 204-1) This invention relates to a method of chromiumplating, and more particularly to a method of plating bright and heavydeposits of chromium.

Considerable research work has been done with the object of determiningthe most efiective temperature of the chromium plating bath and theproper current density at which the bath should be operated, and itappears that the prevalent opinion is that the temperature of the bathordinarily must not exceed 55 C. and that the current density should bemaintained low in order to obtain good deposits.

An object of the present invention is to provide an effective andeflicient method of chr0- mium plating.

In accordance with one embodiment of the invention, the relationshipbetween the temperature of the bath, the density of the plating current,

and the ratio between cathode area and anode Referring now tothe drawingin which the ordinates represent temperature in degrees centigrade andthe abscissas represent current density at the cathode area, the totalenclosed area is the area in which bright deposits of chromium may beobtained. In the lower left hand corner of this area there is a smallerarea X delimited in part by a solid line A, and in part by the dash lineB in which the relationship between the cathode area and anode area ofthe plating electrodes is unimportant and it was heretofore consideredthat chromium plating outside of this area was not practical. We havefound, however, chromium plating may be carried out in a much largerarea, utilizing plating bath temperatures as high as 90 0., by properlyproportioning the areas of the cathode and anode. In some cases, it wascustomary in prior practice to em-= of cathode surface with a cathodeanode area ratio of 1 to 3. The solid line C represents the optimumtemperature-current density relationships although, as stated above,satisfactory.results may be obtained within the area enclosed by thedash line B. Beneath the enclosed area the deposits tend to become milkyin appearance, while above the enclosed area nodules are formed, andboth aboveand below this area excessive evolution of gas takes place.

While no definite theory has been established as to why a relatively lowanode area must be used at the higher temperatures, it appears that asthe'conductivity of the bath increases with increase of temperature, thedecrease in anode area reestablishes the relationship existing at thelower temperatures with a larger anode area. The ratio of 1 to 3 betweenthe cathode anode areas is not a fixed quantity and varies with theconfiguration of the parts which serve as cathodes during the platingoperation as well as with the spacing of the parts, but the ratio ismaintained under 1 to 5 for satisfactory results. It has been found thatin some cases where nodules are formed during the plating operation, acloser spacing of the articles may overcome the defect. The mostsatisfactory way of determining the proper anode area for articles of atype which have not been plated before is to begin with a large numberoi removable anodes and then remove anodes until good plating conditionsare arrived at. a given number, size and configuration of parts, thesame anode area may be used for subsequent similar operations, althoughit has been found at times that due to the change of the sulphatechromicacid ratio of the bath, it becomes necessary to remove additional anodesafter the bath has been in operation for some time.

conditions which are satisfactory for applying flash coatings ofchromium of the order of .00001 inch in thickness are not practical forheavy deposits. The method of this invention has been found particularlyeflicacious for apply-' ing smooth and heavy chromium deposits of theorder of .001" in thickness. Due to the porosity of chromium platingdeposits, it is customary to undercoat chromium with a layer of copperand a layer of nickel to protect the surface against corrosion, afterwhich a light or flash coating of chromium is deposited on the nickelundercoat. By means of the present invention, the chromium plate may bemade sufliciently heavy to Once the anode area is established forprotect the underlying surface. In coating zinc, or zinc base alloys,protection against corrosion is secondary, and a single coating ofchromium will suiiice, although it is desirable that such coating shouldbe heavier than whena copper and nickel undercoating is used.

In chromium plating on zinc or zinc alloy parts,

it has been found that the ratio of sulphate radical to chromic acid ismuch less critical when plating'at a temperature of approximately 60 C.and a current density 01' approximately 3 amperes per square inch 01.cathode surface than when plating in the area X of the accompanyinggraph.

The luster oi the article after plating will depend on the condition orthe surface .before plating, and it the luster is desired, the surfaceto be plated is first. subjected to a bufling or brushing operation toprovide a polished surface on which to deposit the chromium. Incomecases it may also be desirable to bill! and color the chromiumdeposit after the plating operation.

It will be understood that the embodiment of the invention hereindescribed is merely illustrative and that many changes and modificationsmay be made without departing from the spirit and scope of theinvention.

What is claimed is:

1. The process of electrodepositing a bright heavy coating of chromiumon an article from an aqueous bath of the chromic acid type, containingabout 250 grams of chromic acid and'l to 5 grams of sulphate per liter,which comprises passing a current of at least 2 amperes per square inchto the article to be plated as a cathode, maintaining the temperature ofthe both between 40 and 90 C. and adjusting the cathode-anode area ariasratioontheo'rderofltoiltoobtainasmooth deposit.

2. The process or plating a bright heavycoatingoichromiumonanarticleinanaq'ueousbath of the chromic acid type,containing about 250 gramsofchromicacidandltoigramsofsub phate perliter, which comprises passing a current 01' suflicient intensity toproduce a bright deposit to the article to be plated asa cathode.

correlating the temperature of the bath, with the current density in amanner such that the current density and temperature vary together inthe same sense and always within the area delimited by the line B in theaccompanying graph for the given temperature, while maintaining the.

cathode-anode area ratio under 1-5.

3. The process of plating a bright heavy coatingoichromiumonanarticleinan aqueousbath of the chromic acid type containing about250 gramsoi'chromicacid'andltoSgramsotsuL' phate per liter, whichcomprises passing a current of suilicient intensity to produce'a brightde-" MORRIS BROWN. ARTHUR H. WAGNER. WILLIAM YoNxmm.

